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CE 6405 SOIL MECHANICS

UNIT I INTRODUCTION

Part A

1. Distinguish between Residual and Transported soil.

2. Give the relation between γsat, G, γw and e

3. A compacted sample of soil with a bulk unit weight of 19.62 kN/m3

has a water content of 15 per cent. What are its dry density, degree of

saturation and air content? Assume G = 2.65.

4. What are all the Atterberg limits for soil and why it is necessary?

5. Define sieve analysis and sedimentation analysis and what is

the necessity of these two analysis?

6. Two clays A and B have the following properties.

Atterberg limits Clay A Clay B

Liquid limit 44 % 55%

Plastic limit 29% 35%

Natural water content 30% 50%

Which of the clays A or B would experience larger settlement under

identical loads? Why?

7. Determine the maximum possible voids ratio for a uniformly graded

sand of perfectly spherical grains.

8. What is a zero air voids line? Draw a compaction curve and show

the zero air voids line.

9. What is porosity of a given soil sample?

10. What is water content in given mass of soil?

11. Define : Porosity and Void ratio

12. Define effective size of particle in sieve analysis

13. Write any two engineering classification system of soil.

14. List any one expression for finding dry density of soils.

15. Define water content and compaction

16. What are the laboratory methods of determination of water

content?

17. Define degree of saturation and shrinkage ratio

18. Define specific gravity and density index.

19. What do understand from grain size distribution?

20. What are consistency limits of soil?

21. Define plasticity index, flow index and liquidity index.

22. What are the methods available for determination of in-situ

density?

23. What is the function of A-line Chart in soil classification?

24. Write the major soil classifications as per Indian Standard

Classification System.

25. Differentiate standard proctor from modified proctor test.

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PART - B (16 marks)

1. Write down a neat procedure for determining water content and

specific gravity of a given soil in the laboratory by using a pycnometer.

2. Sandy soil in a borrow pit has unit weight of solids as 25.8 kN/m3,

water content equal to 11% and bulk unit weight equal to 16.4 kN/m3.

How many cubic meter of compacted fill could be constructed of 3500

m3 of sand excavated from borrow pit, if required value of porosity in

the compacted fill is 30%. Also calculate the change in degree of

saturation.

3. The following data on consistency limits are available for two soils A

and B.

Find which soil is

(i) More plastic.

(ii) Better foundation material on remoulding.

(iii) Better shear strength as function of water content.

(iv) Better shear strength at plastic limit. (AUC Apr/May 2010)

Classify the soil as per IS classification system. Do those soils

have organic matter?

4. 4. By three phase soil system, prove that the degree of saturation

S (as ratio) in terms of mass unit weight (γ), void ratio (e), specific

gravity of soil grains (G) and unit weight of water (γw) is given

by the expression

5. The mass of wet soil when compacted in a mould was 19.55 kN.

The water content of the soil was 16%. If the volume of the mould was

0.95 m3. Determine (i) dry unit weight, (ii) Void ratio, (iii) degree of

saturation and (iv) percent air voids. Take G = 2.68.

6. In a hydrometer analysis, the corrected hydrometer reading in a

1000 ml uniform soil suspension at the start of sedimentation was 28.

After a lapse of 30 minutes, the corrected hydrometer reading was 12

and the corresponding effective depth 10.5 cm. the specific gravity of

the solids was 2.68. Assuming the viscosity and unit weight of water at

the temperature of the test as 0.001 Ns/m2 and 9.81 kN/m

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respectively. Determine the weight of solids mixed in the suspension,

the effective diameter corresponding to the 30 minutes reading and the

SI.No. Index Soil A Soil B

1 Plastic limit 16% 19%

2 Liquid limit 30% 52%

3 Flow index 11 06

4 Natural water content 32% 40%

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percentage of particle finer than this size.

7.An earthen embankment of 106 m

3 volume is to be constructed with a

soil having a void ratio of 0.80 after compaction. There are three

borrow pits marked A, B and C having soils with voids ratios of 0.90,

0.50 and 1.80 respectively. The cost of excavation and transporting the

soil is Rs 0.25, Rs 0.23 and Rs 0.18 per m3 respectively. Calculate the

volume of soil to be excavated from each pit. Which borrow pit is the

most economical? (Take G = 2.65)

8. A laboratory compaction test on soil having specific gravity equal to

2.67 gave a maximum dry unit weight of 17.8 kN/m3 and a water

content of 15%. Determine the degree of saturation, air content and

percentage air voids at the maximum dry unit weight. What would be

theoretical maximum dry unit weight corresponding to zero air voids at

the optimum water content?

9. A soil sample has a porosity of 40 per cent. The specific gravity of

solids is 2.70. Calculate

i) Voids ratio

ii) Dry density and

iii) Unit weight if the soil is completely saturated

10 .A soil has a bulk unit weight of 20.11 KN/m3 and water content of

15 percent. Calculate the water content of the soil partially dries to a

unit weight of 19.42 KN/m3 and the voids ratio remains unchanged.

11. Explain Standard Proctor Compaction test with neat sketches

12. Soil is to be excavated from a barrow pit which has a density of

17.66kN/m3 and water content of 12%. The specific gravity of soil

particle is2.7. The soil is compacted so that water content is 18% and

dry density is16.2 kN/m3. For 1000 cum of soil in fill, estimate.

(i) The quantity of soil to be excavated from the pit in cum

and

(ii)The amount of water to be added. Also determine the void

ratios of the soil in borrow pit and fill.

13.Explain all the consistency limits and indices.

14.Explain in detail the procedure for determination of grain size

distribution of soil by analysis.

15.An earth embankment is compacted at a water content of 18% to a

bulk density of 1.92 g/cm3. If the specific gravity of the sand is 2.7, find

the void ratio and degree of saturation of the compacted embankment

16.Explain the procedure for determining the relationship between dry

density and moisture content by proctor compaction test.

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UNIT 2- SOIL WATER AND WATER FLOW

PART - A (2 marks)

1. What are the different types of soil water?

2. List out the methods of drawing flow net.

3. What is meant by total stress, neutral stress and effective stress?

4. What is meant by capillary rise in soil and how it affects the stress

level in soils?

5. Prove that effective stress in soil mass is independent of variation in

water table above the above the ground surface.

6. State and explain Darcy’s law

7. What is quick sand? How would you calculate the hydraulic gradient

required to create quick sand conditions in a sample of sand?

8. For a homogeneous earth dam 52 m high and 2 m free board, a flow

net was constructed and following results were obtained: Number of

potential drops = 25; Number of flow channels = 4 .

9. Calculate the discharge per metre length of the dam if the co-

efficient of permeability of the dam material is 3 x 10-5

m/sec.

10. What is capillary rise?

11. What is surface tension?

12. What are the different forms of soil water?

13. Write down the uses of Flow net.

14. Define Neutral stress.

15. What is seepage velocity?

16. Define soil water and classify the types of soil water.

17. Define Capillarity and permeability.

18. What is surface tension?

19. What is meant by capillary siphoning?

20. Give the relationship between total, neutral and effective stress

21. What are the factors affecting permeability?

22. What are the methods available for determination of permeability in

the laboratory?

23. Define discharge and seepage velocity

24. What are methods of determination of permeability in the field?

25. Define seepage pressure and flow net

26. What is quick sand condition?

PART - B (16 marks)

1. The water table in a deposit of sand 8 m thick is at a depth of 3 m

below the ground surface. Above the water table, the sand is saturated

with capillary water. The bulk density of sand is 19.62 kN/m3. Calculate

the effective pressure at 1m, 3m and 8m below the ground surface.

Hence plot the variation of total pressure, neutral pressure and effective

pressure over the depth of 8m

2. Write down the procedure for determination of permeability by

constant head test in the laboratory.

3. Compute the total, effective and pore pressure at a depth of 20 m

below the bottom of a lake 6 m deep. The bottom of lake consists of

soft clay with a thickness of more than 20 m. the average water

content of the clay is 35% and specific gravity of the soil may be

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assumed to be 2.65.

4. What will be the ratio of average permeability in horizontal direction

to that in the vertical direction for a soil deposit consisting of three

horizontal layers, if the thickness and permeability of second layer are

twice of those of the first and those of the third layer twice those of

second?

5. The subsoil strata at a site consist of fine sand 1.8 m thick overlying

a stratum of clay 1.6 m thick. Under the clay stratum lies a deposit of

coarse sand extending to a considerable depth. The water table is 1.5

m below the ground surface. Assuming the top fine sand to be

saturated by capillary water, calculate the effective pressures at

ground surface and at depths of 1.8 m, 3.4 m and 5.0 m below the

ground surface. Assume for fine sand G = 2.65, e = 0.8 and for coarse

sand G = 2.66, e = 0.5. What will be the change in effective pressure at

depth 3.4 m, if no capillary water is assumed to be present in the fine

sand and its bulk unit weight is assumed to be 16.68 kN/m3. The unit

weight of clay may be assumed as 19.32 kN/m3

6. In a constant head permeameter test, the following observations

were taken. Distance between piezometer tappings = 15 cm, difference

of water levels in piezometers = 40 cm, diameter of the test sample = 5

cm, quantity of water collected = 500 ml, duration of the test = 900 sec.

determine the coefficient of permeability of the soil. If the dry mass of

the 15 cm long sample is 486 g and specific gravity of the solids is

2.65. Calculate seepage velocity of water during the test.

7. A foundation trench is to be excavated in a stratum of stiff clay, 10m

thick, underlain by a bed of coarse sand (fig.1.). In a trial borehole the

ground water was observed to rise to an elevation of 3.5m below

ground surface. Determine the depth upto which an excavation can be

safely carried out without the danger of the bottom becoming unstable

under the artesian pressure in the sand stratum. The specific gravity of

clay particles is 2.75 and the void ratio is 0.8. if excavation is to be

carried out safely to a depth of 8m, how much should the water table

be lowered in the vicinity of the trench?

8. The following data were recorded in a constant head permeability

test. Internal diameter of permeameter = 7.5cm Head lost over a

sample length of 18cm = 24.7cm Quantity of water collected in 60 Sec

= 626 ml Porosity of soil sample was 44% Calculate the coefficient of

permeability of the soil. Also determine the discharge velocity and

seepage velocity during the test

9. Explain the falling head permeability test

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10. What are the applications of flow net and explain briefly?

11. Determine the effective stress at 2m, 4m, 6m, 8m and 10m is a soil

mass having γs =21 KN/m3 Water table is 2m below ground surface.

Above water table there is capillary rise upto ground surface. Also

draw total stress diagram up to 10m.

12. A stratified soil deposit is shown in Fig.1. Along with the coefficient

of permeability of the individual strata. Determine the ratio of KH and

KV. Assuming an average hydraulic gradient of 0.3 in both horizontal

and vertical seepage, Find Discharge value and discharge velocities in

each layer for horizontal flow and Hydraulic gradient and loss in head

in each layer for vertical flow

13. Explain any four methods of obtaining flow nets.

14. The discharge of water collected from a constant head

permeameter in a period of 15 minutes is 500 ml. the internal diameter

of the permeameter is 5 cm and the measured difference in head

between two gauging points 15 cm vertically apart is 40 cm. calculate

the coefficient of permeability. If the dry weight of the 15 cm long

sample is 486 gm and the specific gravity of the solids is 2.65,

calculate the seepage velocity.

15. Explain in detail the laboratory determination of permeability using

constant head method and falling head method.

16. Explain in detail the procedure for drawing the phreatic line for an

earthen dam.

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UNIT 3 – STRESS DISTRIBUTION, COMPRESSIBILITY AND

SETTLEMENT

Part A

1. Write down Boussinesque equation for finding out the vertical

stress under a single concentrated load.

2. Define normally consolidated clays and over consolidated clays

3. Explain the method of estimating vertical stress using Newmark’s

influence chart

4. What are the assumptions made in Terzaghi’s one dimensional

consolidation theory?

5. What is the use of influence chart in soil mechanics?

6. Differentiate between ‘Compaction’ and ‘Consolidation’

7. Write down the use of influence charts.

8. What are isochrones?

9. When a soil mass is said to be homogeneous?

10. What are isobars?

11. Differentiate Consolidation and Compaction.

12. List the components of settlement in soil.

13. What are the two theories explaining the stress distribution on soil?

14. What is oedometer?

15. What is geostatic stress and pre-consolidation pressure?

16. What are the applications of Boussinesque equation?

17. What is a pressure bulb and Newmark’s Chart?

18. Write the equation for stress in soil due to a uniformly loaded

circular area

19. Write the equation for stress in soil due to a line load.

20. Write the equation for stress in soil beneath a corner of a uniformly

loaded rectangular area.

21. Write the Westergaard’s equation for stress beneath a

concentrated point load

22. Define co-efficient of compressibility and compression index

23. What are the methods to determine co-efficient of consolidation?

24. What are the factors influencing consolidation?

25. Define Over consolidation ratio and creep

PART – B (16 marks)

1.A water tank is supported by a ring foundation having outer diameter

of 10 m and inner diameter of 7.5 m. the ring foundation transmits

uniform load intensity of 160 kN/m2. Compute the vertical stress

induced at depth of 4 m, below the centre of ring foundation, using

Boussinesque analysis and Westergaard’s analysis, taking µ = 0.

2. A stratum of clay with an average liquid limit of 45% is 6m thick. Its

surface is located at a depth of 8m below the ground surface. The

natural water content of the clay is 40% and the specific gravity is 2.7.

Between ground surface and clay, the subsoil consists of fine sand.

The water table is located at a depth of 4m below the ground surface.

The average submerged unit weight of sand is 10.5 kN/m3 and unit

weight of sand above the water table is 17 kN/m3. The weight of the

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building that will be constructed on the sand above clay increases the

overburden pressure on the clay by 40 kN/m2. Estimate the

settlements of the building.

3. A concentrated point load of 200 kN acts at the ground surface.

Find the intensity of vertical pressure at a depth of 10 m below the

ground surface and situated on the axis of the loading. What will be

the vertical pressure at a point at a depth of 5 m and at a radial

distance of 2 m from the axis of loading? Use Boussinesque analysis.

4. Explain with a neat sketch the Terzhaghi’s one dimensional

consolidation theory.

5.The load from a continuous footing of width 2m, which may be

considered to be strip load of considerable length, is 200 kN/m2.

Determine the maximum principal stress at 1.5m depth below the

footing, if the point lies (i) directly below the centre of the footing, (ii)

directly below the edge of the footing and (iii) 0.8m away from the edge

of the footing.

6.What are different components of settlement? Explain in detail

7.In a laboratory consolidometer test on a 20 mm thick sample of

saturated clay taken from a site, 50% consolidation point was reached

in 10 minutes. Estimate the time required for the clay layer of 5 m

thickness at the site for 50% compression if there is drainage only

towards the top. What is the time required for the clay layer to reach

50% consolidation if the layer has double drainage instead of single

drainage

8.What are the various components of a settlement? How are these

estimated?

9.Explain the Newmark’s influence chart in detail.

10. How will you determine preconsolidation pressure?

11.How will you determine coefficient of compression index (CC) from

an oedomoter test?

12.An undrained soil sample 30cm thick got 50% consolidation in 20

minutes with drainage allowed at top and bottom in the laboratory. If

the clay layer from which the sample was obtained is 3m thick in field

condition, estimate the time it will take to consolidate 50% with double

surface drainage and in both cases, consolidation pressure is uniform.

13.Derive Boussinesque equations to find intensity of vertical pressure

and tangential stress when a concentrated load is acting on the soil.

14.Explain the assumptions made by Boussinesque in stress

distribution on soils.

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A line load of 100 kN/m run extends to a long distance. Determine the

intensity of vertical stress at a point, 2 m below the surface and Directly

under the line load and At a distance 2 m perpendicular to the line. Use

Boussinesq’s theory.

15.Explain in detail the laboratory determination of co-efficient of

consolidation.

16.A layer of soft clay is 6 m thick and lies under a newly constructed

building. The weight of sand overlying the clay layer produces a

pressure of 2.6 kg/cm2 and the new construction increases the

pressure by 1.0 kg/cm2. If the compression index is 0.5. Compute the

settlement. Water content is 40% and specific gravity of grains is

2.65

Unit 4 - SHEAR STRENGTH

Part A

1. Write down the Mohr’s-Coulomb failure envelope equation.

2. Why triaxial shear test is considered better than direct shear test?

3. What are different types of triaxial compression tests based on

drainage conditions?

4. Explain the Mohr–Coulomb failure theory.

5. State the principles of Direct shear test?

6. What is the effect of pore pressure on shear strength of soil?

7. How will you find the shear strength of cohesionless soil?

8. List out the types of shear tests based on drainage.

9. What is shear strength of soil?

10. Write down the Coulomb’s expression for shear strength.

11. How will you find the shear strength of cohesive soil?

12. What are the advantages of Triaxial Compression Test?

13. Define ‘angle of repose’ of soil

14. Write the expression for coulomb’s law.

15. Define shear strength and failure envelope

16. What are the shear strength parameters?

17. Define Cohesion and stress path

18. What is angle of internal friction?

19. What are the various methods of determination of shear strength in

the laboratory?

20. Write the differential equation of deflection of a bent beam?

21. What are the disadvantages of direct shear test?

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22. What are the types of triaxial test based on drainage conditions?

23. When is vane shear test adopted?

24. Sketch the Mohr’s circle for total and effective stresses for

undrained triaxial test

25. Sketch the failure envelope for drained triaxial test

PART – B (16 marks)

1.Obtain the relationship between the principal stresses in triaxial

compression test using Mohr-Coulomb failure theory

Two identical soil specimens were tested in a triaxial apparatus. First

specimen failed at a deviator stress of 770 kN/m2 when the cell

pressure was 2000 kN/m2. Second specimen failed at a deviator stress

of 1370 kN/m2 under a cell pressure of 400 kN/m

2. Determine the value

of c and Φ analytically. If the same soil is tested in a direct shear

apparatus with a normal stress of 600 kN/m2, estimate the shear stress

at failure

2. A saturated specimen of cohesion less sand was tested in triaxial

compression and the sample failed at a deviator stress of 482 kN/m2

when the cell pressure was 100 kN/m2 under the drained conditions.

Find the effective angle of shearing resistance of sand. What would be

the deviator stress and the major principal stress at failure for another

identical specimen of sand, if it is tested under cell pressure of 200

kN/m2. Use either Mohr’s circle method or analytical method

Write down a step by step procedure for determination of cohesion of a

given clayey soil by conducting unconfined compression test.

3. Explain with neat sketches the procedure of conducting direct shear

test. Give its advantages over other methods of finding shear strength

of soil.

4. Write a brief critical note on unconfined compression test

5.What are the advantages and disadvantages of triaxial compression

test.

6.A vane, 10 cm long and 8 cm in diameter, was pressed into soft clay

at the bottom of a bore hole. Torque was applied and gradually

increased to 45 N-m when failure took place. Subsequently, the vane

rotated rapidly so as to completely remould the soil. The remoulded

soil was sheared at a torque of 18 N-m. Calculate the cohesion of the

clay in the natural and remoulded states and also the value of the

sensitivity.

7.Describe the triaxial shear test. What are the advantages of triaxial

shear test over the direct shear test.

8.Explain the Triaxial compression test to determine the shear strength

of soil

9.Explain drained behavior of clay with reference to shear strength.

10.Explain the direct shear test to determine the shear strength of soil

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11.Explain the Mohr-Coulomb failure theory

12.Explain with neat sketch Direct Shear method of finding Shear

Strength.

13.The following data were obtained in a direct shear test. Normal

pressure 20 kN/m2, Tangential pressure = 16 kN/m

2, Angle of internal

friction = 200, Cohesion = 8 kN/m

2. Represent the data by Mohr’s circle

and compute the principal stresses and the direction of principal planes

Compare the merits and demerits of triaxial compression test

14.A particular soil failed under a major principal stress of 300 kN/m2

with a corresponding minor principal stress of 100 kN/m2. If for the

same soil, the minor principal stress had been 200 kN/m2. Determine

what the major principal stress would have been if (i) Φ = 300 and ii) Φ

= 00

15.A Cylindrical specimen of dry sand was tested in a triaxial test.

Failure occurred under a cell pressure of 1.2 kg/cm2 and at a deviator

stress of 4.0kg/cm2. Find Angle of shearing resistance of the soil.

Normal and shear stresses on the failure plane. The angle made by

the plane with the minor principal plane. The maximum shear stress on

any plane in the specimen at the instant of failure.

16.Explain in detail the determination of shear strength using

unconfined compression test

17.Explain in detail the determination of shear strength using vane

shear test.

18.Explain the shear strength behavior of cohesive and cohesionless

soils under different drainage condition in a triaxial test

Unit 5 – SLOPE STABILITY

PART – A (2 marks)

1.Differentiate finite slope and infinite slope.

2.Write down the expression for factor of safety of an infinite slope in

case of cohesion less soil.

3.List out any two slope protection methods.

4.What do you mean by Tension crack?

5.Define critical surface of failure

6.What are different factors of safety used in the stability of slopes?

7.What is a stability number? What are the uses of stability charts?

8.State the two basic types of failure occurring in finite slopes.

9.What is a slide?

10.What are the different types of Slope failure?

11.State some of the Slope protection measures

12.Mention the types of slopes in soil.

13.Define stability number.

14.What are the types of slopes?

15.What are the types and causes for slope failure?

16.What are the various methods of analysis of finite slopes?

17.Define factor of safety and critical depth.

18.Define stability number.

19.How does tension crack influence stability analysis?

20.What are the various slope protection measures?

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PART – B (16 marks)

1.Explain the procedure to calculate the factor of safety of a finite slope

possessing both cohesion and friction (c - Φ) by method of slices

2.A slope is to be constructed in a soil for which c = 0 and Φ = 36°. It is

to be assumed that the water level may occasionally reach the surface

of a slope with seepage taking place parallel to the slope. Determine

the maximum slope angle for a factor of safety 1.5, assuming a

potential failure surface parallel to the slope. What would be the factor

of safety of the slope, constructed at this angle, if the water table

should be below the surface? The saturated unit weight of the soil is 19

kN/m3.

3.A new canal is excavated to a depth of 5 m below ground level

through a soil having the following characteristics: C = 14 kN/m2; Φ =

15°; e = 0.8 and G = 2.70. The slope of banks is 1 in 1. Calculate the

factor of safety with respect to cohesion when the canal runs full. If it is

suddenly and completely emptied, what will be the factor of safety?

4.Write down the procedure for determining the factor of safety of a

given slope by friction circle method

5.A canal is to be excavated to a depth of 6m below ground level

through a soil having the following characteristics c = 15 kN/m2, Φ =

20°, e = 0.9 and G = 2.67. The slope of the banks is 1 in 1. Determine

the factor of safety with respect to cohesion when the canal runs full.

What will be the factor of safety if the canal is rapidly emptied

completely?

6.Explain with neat sketches the Bishop’s method of stability analysis

7.What are different types of slope failures? Discuss the various

methods for improving the stability of slopes.

8.An embankment 10 m high is inclined at 35 to the horizontal. A

stability analysis by the method of slices gave the following forces: ∑N

= 900kN, ∑T = 420kN, ∑U = 200kN. If the length of the failure arc is

23.0 m, find the factor of safety. The soil has c = 20kN /m2 and Φ =

15o.

9.Explain the Swedish slip circle method in detail

10.Explain Taylor’s stability number and its applicability.

11.Explain in detail the friction circle method of stability analysis for

slopes with sketch.

12.Explain any four methods of slope protection

13.A cut 9 m deep is to be made in clay with a unit weight of 18 kN/m3

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and cohesion of 27 kN/m2. A hard stratum exists at a depth of 18 m

below the ground surface. Determine from Taylor’s charts if a 300 slope

is safe. If a factor of safety of 1.50 is desired, what is a safe angle of

slope?

14.Explain in detail the various methods to protect slopes from failure.